This invention relates to hydraulic brake pressure regulators for hydraulic brake systems of motor vehicles, and, more particularly, to brake pressure regulators having a family of characteristic curves dependent upon a vehicular parameter such as outlet pressure as a function of inlet pressure.
Such brake pressure regulators are used to obtain braking performance for a motor vehicle which is independent of variable vehicular parameters, such as the load on the rear axle or the deceleration of the vehicle, to provide as nearly as possible a constant braking performance for the vehicle.
It is common knowledge that when a motor vehicle is braked the front axle becomes more heavily loaded, in comparison with its static load, and the rear axle is correspondingly unloaded. As is known, optimum braking of a vehicle is obtained when the front and rear axles require the same frictional brake contact at particular road conditions and at a particular speed. This is the case when, for instance, the front and rear axles, on braking, simultaneously reach the transition region for locking.
When the dimensions, weight and location of the center of gravity of the vehicle are known, the optimum distribution of the braking force between the front and rear axles may be calculated. This ideal braking-force or braking-pressure distribution represented by P.sub.1 (front) and P.sub.2 (rear), referred to the weight G of the vehicle, is shown qualitatively in FIG. 1 for the "full load" condition by the upper thin solid line and for the "empty load+driver" condition by the lower solid line. It may be seen that continuous nonlinear characteristic curves are involved here. A separate curve (not shown), lying between these upper and lower boundary lines, applies to each intermediate load condition of the vehicle.
Locking of the rear wheels before the front wheels are locked should be avoided in every load condition because it results in unstable driving performance. To avoid such overbraking of the rear axle, the distribution of braking forces to the front and rear axles must be selected in designing a hydraulic brake system so that the effective braking force on the rear axle is never higher than the braking force shown in FIG. 1 for the case of ideal braking.
It would thus be possible to avoid overbraking of the rear axle in simple fashion by providing a fixed braking-force distribution which could, for example, be aimed at making the real braking-force distribution at braking, a=0.8, a point of the ideal braking-force distribution for the load case "empty load+driver" i.e., the lower thin line shown in FIG. 1. In such a fixed braking-force distribution, the straight line represented in FIG. 1 as a heavy solid line would then be produced.
It is easy to see that, with such a fixed braking-force distribution, the ideal conditions would be present only for this special load case and for the assumed braking a=0.8, whereas with greater loading of the vehicle or with a smaller demand for frictional contact conditions far from ideal braking performance would result.
In order to at least approximate the actual braking-force distribution of a motor vehicle to the ideal braking-force distribution, pressure regulators having a bent characteristic line with a knee position which varies according to the load, are disclosed, for example in DE-AS No. 1,655,003, FIG. 3, and are already included in many vehicles. Typical characteristic bent lines of such a known brake pressure regulator are indicated by dashed lines in FIG. 1. Even when such a known load-dependent brake pressure regulator is used, the real braking-force distribution is still clearly far from the ideal distribution.
A better approximation of the ideal braking-force distribution is provided by hydraulic brake regulators whose family of characteristic lines consists of a group of bent straight lines in which both the slope and the knee are variable in accordance with the load. Such brake pressure regulators are disclosed in, for example, DE-OS No. 2,708,941 and DE-OS No. 2,923,018. The two boundary lines of such known brake pressure regulators, which might alternatively be called characteristic line regulators having a load-dependent knee, are indicated by dash-dotted lines in FIG. 1. But even these known hydraulic brake pressure regulators having their degressive characteristic lines still leave a great deal to be desired with respect to their adaptability to ideal braking-force distribution. In addition, they are comparatively complicated in design. One known brake pressure regulator of this type (DE-OS No. 2,923,018) works, for example, like known characteristic regulators (DE-OS No. 1,780,560) using the principle of the scale balance, in which the effective lever lengths of a scale balance cooperating with two piston members are automatically varied as a function of the vehicle load. This brake pressure regulator is not only mechanically complicated, but likewise has a comparatively large size.
The object of the invention is to provide an improved hydraulic brake pressure regulator for the hydraulic brake system of a motor vehicle, having a family of characteristic curves dependent upon a vehicle parameter such as outlet pressure as a function of inlet pressure.
Another object of the invention is to provide a hydraulic pressure regulator which not only provides a family of characteristic curves adapted as closely as possible to the ideal braking-force distribution but also has comparatively low construction and manufacturing costs and small spatial requirements.